Signals are transmitted acoustically in the prior art with air or water as a medium. We do not known of any prior patent art, publications or apparatus which disclose transmission of a signal over a gaseous or liquid medium as a double-sideband amplitude modulated carrier, with rejection of noise from the medium in the receiving apparatus by sampling and reconstruction means to produce the modulated carrier substantially free superimposed noise. The sampling principle is fully described by P. F. Panter in "Modulation, Noise, and Spectral Analysis", McGraw-Hill 1965, pages 506 to 547 inclusive. The reduced Nyquist frequency for a double-sideband amplitude modulated carrier is disclosed in U.S. Pat. No. 4,178,553, class 325/487, "Sampling Modulated Waves", issued Dec. 11, 1979 to Fisher et al.